The molecular events associated with active transport of Na ion and the (Na ion plus K ion)-ATPase have been investigated utilizing a rapid-quenching technique to study the transient kinetics of the phosphorylated enzyme. These studies have demonstrated the existence of slowly convertible states of the enzyme that lie outside the main reaction pathway. Studies utilizing a new technique for measuring ouabain binding under equilibrium conditions have demonstrated that lithium ions bind to more restricted sites that those accessible to K ion.